1. Field
The present invention is concerned with novel cross-linkable, photoactive polymer materials having 3-aryl-acrylic acid esters and amides. The present invention also provides for the use of such materials as orienting layers for liquid crystals and for the production of non-structured and structured optical elements, and multi-layer systems.
2. Description
The orienting layer is of special significance in (electro-optical) liquid crystal indicators. It serves to guarantee a symmetrical and disturbance-free direct orientation of the molecular axis.
Uniaxially-rubbed polymer orienting layers such as polyamide are usually used for orienting liquid crystal molecules in liquid crystal indicators ("LCD's"). In this procedure, the direction of rubbing provides the direction of orientation. However, a number of serious disadvantages which can severely influence the optical quality of liquid crystal indicators, are associated with rubbing. For example, dust produced by rubbing can lead to optically defective sites in the display. At the same time, the polymer layer becomes electrostatically charged. In the case of thin film transistor ("TFT")-TN-LCD's, such eletrostatic charging can result in damage to the thin layer transistors which lie below it. For these reasons, the yield of optically perfect displays in LCD production has hitherto not been optimal.
A further disadvantage of rubbing is that it is not possible to produce structured orienting layers in a simple manner, since orientation direction cannot be locally varied by rubbing. Thus, only essentially uniformly directed layers of large area can be produced by rubbing. Structured orienting layers are, however, of great interest in many fields of display technology and integrated optics. For example, the viewing angle dependence of twisted nematic ("TN")-LCD's can be produced with such orienting layers.
Orienting layers in which the direction of orientation can be provided by irradiation with polarized light have been known for some time. Thus, problems inherent in rubbing can be avoided. In addition, the possibility exists of providing local variations of the direction of orientation and thus of structuring the orienting layer.
One possibility for the structured orientation of liquid crystals utilizes the ability of certain dye molecules to isomerize in order to induce a differential direction photochemically by irradiation with polarized light of suitable wavelength. This is achieved, for example, by admixing an orienting polymer and a dye, which is then irradiated with polarized light. Such a guest/host system is described, for example, in U.S. Pat. No. 4,974,941, the contents of which are herein incorporated by reference. In this system azobenzenes are incorporated in polyimide orienting layers and are subsequently irradiated with polarized light. Liquid crystals which are in contact with the surface of a thus-exposed layer are oriented correspondingly to this preferential direction. This orienting procedure is reversible, that is, the already inscribed direction of orientation can be reversed by repeating the irradiation of the layer with light of a second direction of polarization. Since this re-orientation procedure can be repeated at will, orienting layers on this basis are not very suitable for use in LCD's.
A further possibility for the production of high resolution orienting patterns in liquid crystalline layers is described in Jpn. J. AppI. Phys., 31:2155 (1992). In this procedure, dimerization of polymer-bound photoreactive cinnamic acid groups induced by irradiation with linear polarized light is utilized for the structured orientation of liquid crystals. In contrast to the reversible orientation procedure described above, an anisotropic polymer network is built up in the case of the photo-structurable orienting layers described in Jpn. J. AppI. Phys., 31:2155 (1992). These photo-orientated polymer networks are of use primarily where structured or non-structured liquid crystal orienting layers are required. Moreover, such orienting layers can also be used in LCD's, for example, for the production of so-called hybrid layers as are exemplified in European Patent Applications 0 611 981, 0 689 084 and 0 689 065, and in Swiss Patent Application No. 2036/95 which correspond to U.S. application Ser. Nos. 08/194,234, 08/489,865, 08/489,866 and 08/667,687, respectively, the contents of each being herein incorporated by reference. Optical elements such as non-absorptive color filters, linear and circular polarizers, optical retardation layers, etc. can be realized with these hybrid layers from photo-structured orienting polymers and cross-linkable low molecular liquid crystals.
Cinnamic acid polymers which are suitable in principal for the production of such an isotropic cross-linked, photo-structured orienting layers for liquid crystals are described, for example, in European Publication No. 0 611 786, corresponding to U.S. Pat. No. 5,539,074, the contents of which is herein incorporated by reference. These cross-linkable cinnamic acid derivatives are in principle linked to the main chain of the polymer via the carboxyl function of the cinnamic acid (phenylacrylic acid) and a spacer. In these polymers the dimerizable acrylic ester group of the cinnamic acid is always directed from "within" to the spacer and, respectively, polymer backbone, while the aromatic residue is always orientated "outwards" from the polymer backbone.
It has now been found that this method for directing the cinnamic acid in the known photopolymers is by no means optimal. Concurrent photochemical reactions have a damaging effect on the orientation capability. The known cinnamic acid polymers are distinguished by an insufficient photochemical long-term stability. For example, lengthy irradiation with UV light of a pre-finished orienting layer leads to damage of the originally present orientation. Multiple exposures in which the already existing orienting layer having a pre-given inscribed pattern is exposed once more in order to orientate still unexposed regions in a different direction can only be carried out when the previously exposed sites are covered by a mask. Otherwise, the already orientated regions of the layer can completely or partially lose their structure by photochemical side-reactions.
A further disadvantage of a previously used cinnamic acid polymers is that no viewing angle occurs in the case of the orienting surfaces from these materials produced by simple exposure to polarized light. However, especially for use in LCD's, the orienting layer must provide not only the direction of orientation, but also a viewing angle.
In the case of the aforementioned uniaxially rubbed polymer orienting layers this viewing angle is already produced by the rubbing procedure on the polymer surface. When a liquid crystal is brought into contact with such a surface, then the liquid crystal molecules lie inclined rather than parallel to the surface, which thus confers the viewing angle to the liquid crystal. The extent of the viewing angle is thus determined not only by rubbing parameter such as traversing velocity and contact pressure, but also by the chemical structure of the polymer. Viewing angles between 1.degree. and 15.degree. are required for the production of liquid crystal indicators depending on type. The greater viewing angles are required especially for supertwisted nematic ("STN") LCD's in order to avoid the appearance of so-called finger print textures. The rotational direction and the viewing direction in TN and TFT-TN-LCD's are determined by the viewing angle, whereby "reverse twist" and "reverse tilt" phenomena are prevented. While reverse twist in the switched-off state leads to regions having a false direction of rotation which is noticeable optically in a speckled appearance of the indicator, reverse tilt is noticeable with much optical disturbance primarily upon switching the LCD's by angling of the liquid crystals in different directions. Reverse twist can be prevented by doping the liquid crystal mixture with a chiral dopant having a suitable direction of rotation. However, for the suppression of reverse tilt there has hitherto been no alternative possibility to the use of orienting layers having a viewing angle.